Phase lock devices are most commonly used for synchronizing a system to an incoming periodic signal, frequently in connection with inter-system communication where proper interpretation of the communicated signals depends upon their being read within defined timing windows. A phase lock device provides the synchronization by generating a periodic signal which tracks the timing windows of the incoming signal.
A discussion on conventional phase lock devices can be found in "Analysis And Design Of Analog Integrated Circuits" by P. R. Gray et al, John Wiley & Sons, 1977. Basically, a phase lock device comprises a phase comparator, a low pass filter and a voltage controlled oscillator (VCO). The VCO generates a periodic signal whose frequency is adjustable by an externally applied voltage. In tracking a target signal, the VCO output signal is compared with the target signal to produce an error voltage that reflects their phase difference. This error voltage is fed back, via the low pass filter, to the VCO to adjust its output signal so that its phase matches the phase of the target signal.
The periodic signal of a conventional phase lock device is typically generated by a resistor-capacitor type multivibrator. It has a free-running frequency, which is typically the frequency of the VCO in unlocked state. The tracking capability of a phase lock device is usually limited to a frequency range (commonly referred to as the "lock range") surrounding the free-running frequency.
When used to track an incoming signal, it is desirable to tune the free-running frequency of the VCO to the nominal frequency of the incoming signal, so that all deviations of the incoming signal from the nominal frequency may fall within the lock range of the phase lock device.
Unfortunately, tuning a VCO is usually time consuming. This is compounded by the fact that divergence in intrinsic characteristics of its components most frequently necessitate individual tuning of a VCO. Worse yet, temperature sensitivity and age-related performance degradation of the components frequently necessitate a retuning of a VCO.
As a result, there is a need for a digital phase lock device which can be tuned easily. It is further desirable to have a phase lock device that can be tuned under program control.